Some questions in the review queue are there only to test the reviewer. Who chooses those?
From the capital-M Meta FAQ on review audits,
Review audits are chosen automatically. The system isn't perfect, meaning that every so often a post slips through normal community detection, causing the system to expect the wrong type of action against a post.
The answer goes into a little bit more detail about the criteria, for some of these. For example, questions that are "highly-voted with no close votes" can be chosen as tests where the reviewer is expected to keep vote to leave open.